Advice for custom PC

My laptop has basically given up and since I hardly ever moved it anyway, I'm planning to get a desktop. At the moment I like the look of PCspecialist, since they seem to use branded components and have got good reviews as a company. I am particularly looking for advice with regard to the configuration. I'm looking to get a PC that has plenty of room for upgrades to keep it working well for several years.

I'd like to get a relatively small SSD for the OS (windows 7) and then a decent HDD for everything else (Western Digital 640GB 6GB/s perhaps). Is this a good idea? It's not vital, but it would certainly be useful. Would it be relatively easy to just go with the HDD for now and then move the OS onto an SSD later on? Also, what are the current SSDs like in terms of their degradation? And can their original speed be fully restored by formatting or whatever the equivalent is?

With respect to the PSU, would the 350W one be sufficient for that processor, an SSD, HDD and maybe a mid-range GPU? How easy is it to upgrade this component myself?

Finally, with regard to the motherboard, there are two options I'm looking at. The first is the ASUS P8H67-M LX SI and the other is the ASUS P8H67-V (NEW REV 3.0). The first one doesn't have crossfire or USB 3.0. Would getting a PCI-e card for USB 3.0 make up for the fact that the first one lacks it? Besides that, is the difference in quality between these two boards enough to justify the extra £45 given that the motherboard is one of the trickiest part to upgrade?

What's your budget and what will you be using it for? I was considering a SSD for my new build, but decided against it. You could set up two HDDs in RAID0 and get almost the same kind of speeds with much more space for more or less the same amount (maybe even cheaper depending how much space you want).
Installing a SSD later (when they are cheaper) is a good idea and would be relatively painless. As for the PSU it's hard to say but if you are going for a mid-range gaming rig I think 500W is recommended.

I don't have any particularly specialist needs; I'm not a huge gamer, but I do like to play occasionally. My main aim is to get a PC that can be upgraded and last for a long time and so can be adapted to suit a range of needs (the ability to upgrade is the main reason I'm going for a PC over an iMac).

The motherboard is a particular concern. The one above has USB 3.0 and crossfire capabilities, but costs £40 more than the ASUS P8H67-M LX SI, which lacks these. Is that extra £40 worth it, considering the greater difficulty in upgrading the motherboard compared to other components?